1. Technical Field
The present invention relates to a backlight module, in particular to an edge-light backlight assembly using a plurality of light sources.
2. Related Art
A backlight module provides light to pass through a liquid crystal panel, such that users can see images on the liquid crystal panel. The brightness of the backlight module influences the viewing quality of the users, and if the brightness is too low, the users cannot easily see the images on the liquid crystal panel, especially when the background brightness is relatively high.
In a direct-light-type backlight module, since the cold cathode fluorescent lamps (CCFLs) which serve as light sources are disposed above a backlight plate, the brightness can be easily enhanced by increasing the number of the CCFLs. Moreover, there is large space in the direct-light-type backlight module to perform heat dissipation by air cooling, thus avoiding the influence of high temperature.
In an edge-light backlight assembly, generally only one CCFL is utilized. The light projected by the CCFL enters the light guide from a very narrow lateral surface of the light guide. Generally, the width of the lateral surface of the light guide is slightly greater than the diameter of the CCFL, so it is difficult to enhance the brightness of the backlight module by directly adding more CCFLs. If it intends to increase the number of the CCFLs, the thickness of the light guide has to be increased to increase the width of the lateral side of light guide. Since the CCFLs are arranged in parallel corresponding to the longitudinal direction of the lateral surface, and a larger width of the lateral side of light guide allows more CCFLs to be disposed corresponding to the lateral surface of the light guide in parallel, for example, “Reflector for Light Source of Backlight Module” in R.O.C (Taiwan) Utility Model No. 300626. However, the edge-light backlight assembly aims to have a low thickness, so the method of increasing the thickness of the light guide does not meet the development trend of the edge-light backlight assembly.
FIG. 1 is another backlight module 1 having more than one CCFL. Referring to FIG. 1, in the backlight module 1, two CCFLs 2 are respectively disposed corresponding to two opposite lateral surfaces 3a of the light guide 3, and thus it is not required to increase the thickness of the light guide 3. However, the following problems occur due to additionally adding the CCFLs 2.
Firstly, two lateral surfaces 3a corresponding to the two CCFLs 2 are generally corresponding to the top edge and bottom edge of the liquid crystal display (LCD). Therefore, frame widths at the bottom edge and the top edge of the LCD have to be increased simultaneously to accommodate the CCFLs 2. The increased edge occupies additional area at the front side of the LCD, thus reducing the viewing area of the LCD.
Secondly, the two CCFLs 2 of the same specification are disposed at two opposite lateral surfaces 3a of the light guide 3. That is the two lateral surfaces 3a are required to have the same length and width, and thus a backlight surface 3c and a light exiting surface 3d of the light guide 3 are parallel to each other. As a result, a ratio of the light directly leaving the light exiting surface 3d at a small included angle (with respect to a normal line of the light exiting surface 3d) in the light reflected by the backlight surface 3c for the first time is decreased. The light leaving the light exiting surface 3d at a large included angle may leave the light exiting surface 3d after several times of reflection between the light exiting surface 3d and the backlight surface 3c, or the light may directly leave the light exiting surface 3d at a large included angle with respect to the normal line. Therefore brightness in front of the light exiting surface 3d of the light guide 3 is reduced, and adverse effect the efficiency of the CCFLs 2.
Thirdly, the two CCFLs 2 are located at the top edge and the bottom edge of the LCD. In a laptop computer, a shaft is disposed on the bottom edge of the LCD to pivot the LCD to a body of the laptop computer. Cabled and wires for electrically connecting the liquid crystal panel and the CCFL 2 to the electronic circuits in the body pass through the shaft. In a backlight module of a single cold cathode tube 2, the CCFL 2 is located at the top edge of the LCD. In the backlight module having two CCFLs 2, the second CCFL 2 is located at the bottom edge of the LCD. The cables and wires for electrically connecting the liquid crystal panel and the CCFL 2 must bypass the second CCFL 2 firstly and then run through the shaft. The second CCFL 2 results in difficulty in arranging the cables and wires.
Finally, due to thermal convection inside the LCD caused by the two CCFLs 2 at the top edge and the bottom edge, the CCFL 2 at a higher position has a higher temperature. As the two CCFLs 2 have different temperatures, the problem that the luminance of the two CCFLs 2 is not uniform occurs.
To solve the problem of low luminance efficiency of the CCFLs 2, light emitting diodes (LEDs) having higher luminance efficiency are used to replace the CCFLs. The LEDs are arranged on a narrow circuit board in an array, and the higher the arrangement density is, the higher the luminance is. However, the arrangement density of the LED array is still limited, otherwise the generated heat will make the temperature of the LEDs raised rapidly, which impacts the luminance and color rendering of the LEDs. If multiple rows of LED array are adopted to disperse the density the LEDs, the thickness of the light guide is also required to be increased, which does not meets the requirement of edge-light backlight assembly. At the same time, the LEDs are still concentrated at a single side 3a of the light guide 3, and the problem of heat concentration still exists. If the LEDs are disposed at two opposite lateral surfaces 3a of the light guide 2, as shown in FIG. 1, the aforementioned problems still occur.